1. Field of the Invention
The invention relates to optical systems, and in particular it relates to apparatus and methods for reducing optical distortion in optical systems using thermal means.
2. Description of the Prior Art
Essentially all optical systems have imaging imperfections referred to in the art as xe2x80x9caberrationsxe2x80x9d. There are an infinite number of possible aberrations, each varying in a different way with the field position and aperture size of the optical system. Generally it is the low order aberrations that prevail. Depending on the type of optical system, some aberrations are more prevalent than others. Examples of types of aberrations are spherical aberration, coma, astigmatism, field curvature, and distortion. Aberrations can arise for a number of reasons. For example, certain aberrations are inherent in the optical design, which typically assumes perfectly made and positioned optical elements. Other aberrations arise as a result of imperfections in the manufacturing and assembly of the optical system. For example, deviations in the surface curvature, variations in the surface shape (e.g., undulations, grooves, etc.), variations in refractive index, and variations in the spacing, centering and tilt from the ideal design are all potential sources of aberrations.
By virtue of the optical design, the aberrations in the system will have different sensitivities to certain departures from perfectionxe2x80x94i.e., some departures from perfection on some elements contribute more to certain aberrations than other departures. Accordingly, different fabrication and alignment tolerances are placed on different elements.
In many optical systems, distortion is particularly problematic. Distortion is defined as the variation of an image point location from its ideal location as a function of field position. Included in the definition of distortion is magnification error, which is a linear change in the position of points in the image with the distance from the center of the field.
Distortion cannot be corrected at the pupil because each point in the field shares the pupil and any correction at the pupil would produce the same effect over the entire field.
By way of example, in a folded projection optical system, out-of-plane deviations in a folding surface located between the focal planes and the pupil can cause distortion in the image. Distortion arising from the folding surface imperfections can be static, such as those caused by surface figuring errors, or dynamic such those caused by warping of the folding surface by thermal transients. While great pains can be taken to minimize distortion in the optical design, in practice it is very difficult to reduce distortion to tolerable amounts due to fabrication and assembly errors, as well as from environmental effects.
For example, controlling distortion in optical lithography lenses is of extreme importance because different lithography systems are used to image different layers, and small image placement errors ultimately result in the circuit feature produced by one layer failing to connect properly with that of another layer.
An example optical lithography lens system is the Wynne-Dyson system, such as described in U.S. Pat. No. 4,391,494 (hereinafter, xe2x80x9cthe ""494 patentxe2x80x9d), which patent is incorporated herein by reference. The Wynne-Dyson system is a symmetric system having two folding surfaces located close to and equidistant from the object and image planes. Theoretically this design has no distortion of any order because of its symmetrical nature. In practice the folding surfaces, which are internal reflecting surfaces of respective prisms, are the major contributors to image distortion. If the imperfections of the folding surfaces could be made identical then the distortion contributions from the two prisms would be equal and opposite and would cancel. However, in practice figure matching is never perfect and the residual errors contribute substantially to the total distortion observed in the image.
Other sources of distortion include refractive index inhomogeneities in the glasses used in the prisms and the refractive lenses as well as the surface figures of the other refractive components. In practice, distortions as large as 100 nm are possible. The main reason the prism hypotenuse surfaces produce most of the image distortion is because they act as a internally reflecting surface that produces an effect on the reflected wavefront equal to double the index of refraction multiplied by the prism surface imperfection.
One approach used to correct residual amounts of aberrations in optical systems is through the use of adaptive optics. US Pat. No. 4,773,748 to Shih et al. discloses a deformable mirror system for reducing distortion in a projected image. However, this system relies on using a mirror made up of two sheets of material having different coefficients of expansion and an array of electrical resistors to heat the mirror to physically distort the reflective surface. Such a system is not amenable or adaptable for use in certain types of optical systems such as the Wynne-Dyson system because there is no suitable location in the system for such a mirror. Further, there can be no physical contact with the internally reflecting surface of the Wynne-Dyson prism because such contact adversely affects the internal reflection properties of the prism.
While totally eliminating residual distortion from an optical system is probably an impractical pursuit, many optical systemsxe2x80x94and particularly optical lithography systems with refractive elementsxe2x80x94would greatly benefit from apparatus and methods that result in significant reductions in distortion.
A first aspect of the invention is an apparatus for reducing residual distortion in an optical system. The optical system in question has a refractive optical element, such as a prism, having an internally reflecting surface, a surface profile and a refractive index. The apparatus includes a heating/cooling system spaced apart from but in thermal communication with the internally reflecting surface. The heating/cooling system is adapted to create a select temperature distribution in the refractive optical element near the internally reflecting surface to alter the refractive index and/or the surface profile in a manner that reduces the residual distortion.
A second aspect of the invention is a method of reducing distortion in the aforementioned optical system. The method includes arranging a heating/cooling system spaced apart from but in thermal communication with the internally reflecting surface, and then using the heating system to create a select temperature distribution in the refractive optical element near the internally reflecting surface to alter the refractive index and/or the surface profile to reduce residual distortion.